When I've done volunteering abroad, I found that I got more out of it than I think I contributed to the organizations I volunteered with. But, I did become a regular donor when I returned home, so ultimately I think NGOs can derive long-term benefits from overseas volunteers even if the short-term impact is pretty minimal

I've had the same experience. I think volunteering, especially for young people, can change their career trajectory-- usually towards the social impact sector... which could result in long term results.